This invention relates to a system for spatially stabilizing the field of view of a photocell mounted on a camera.
Proper exposure of relatively dark foreground objects using a photocell-controlled camera can be achieved, in the presence of relatively bright background lighting, by arranging for the field of view of the photocell to be inclined downwardly relative to the field of view of the camera objective. In this way, the photocell is preferentially responsive to the brightness of the foreground objects. In such cameras, vertical stabilization of the field of view of the photocell may also be maintained, for proper exposure of foreground objects, in the face of limited changes in elevation angle of the objective. Pitch sensing means for achieving the latter are known in the prior art, e.g., U.S. Pat. Nos. 2,076,482; 3,023,684; and 3,291,996.
While the cameras of the above-noted patents provide correction for pitch, they fail to provide any correction for roll, as for example, where the operator may rotate the camera 90.degree. to take advantage of the dimensional differences in a rectangular film format. For the latter, proper exposure can be obtained only if the camera is also provided with means for maintaining the photocell view in a fixed spatial position in response to rotation of the camera about its optical axis. A camera provided with a system for spatially stabilizing the photocell view in both roll and pitch is disclosed in U.S. Pat. No. 3,442,191. In the last-mentioned patent, the field of view of the photocell is determined by the shape of the meniscus of a liquid partially filling a tube whose axis is parallel to the optical axis of the camera and which is located between the photosensitive surface of a photocell and the scene.
In response to the rotation of the above-noted camera about its optical axis, the attitude of the meniscus and its shape remain unchanged because of the symmetry of the tube about its axis, thus stabilizing the field of view in response to camera roll (rotation of the camera about its optical axis). In response to camera pitch (change in angle of elevation due to rotation of the camera about a pitch axis perpendicular to the optical axis of the camera), the stability of the field of view is dependent on the stability of the curvature of the meniscus since the meniscus acts as a lens located in front of the sensitive surface of the photocell. Unfortunately, the shape of the meniscus changes as the elevation angle of the camera changes primarily because of the nonsymmetry of the tube about the pitch axis. As a consequence, the photocell view shifts with any significant changes in the elevation angle of the camera as measured from the horizontal since the field of view is perturbed to the extent that the shape of the meniscus changes.
The stability of the field of view of the photocell in a camera utilizing the system disclosed in the lastmentioned patent in response to changes in camera elevation angle is thus significantly less than the stability in response to rotation of the camera from a horizontal to a vertical picture taking position. It is therefore an object of the present invention to provide a new and improved system for stabilizing the field of view of a photocell in such a manner as to provide satisfactory and substantially equal stabilization in response to both roll and pitch movement of a camera.